Toughness is the ability of a material to absorb energy and undergo large permanent set without rupture. For many engineering adhesive applications, toughness is often the deciding factor. Plastics, because of their inherent brittleness, have been modified in a variety of ways in efforts to improve the toughness thereof. Epoxy resins, for example, which form a versatile glassy network, exhibit excellent resistance to corrosion and solvents, good adhesion, reasonably high glass transition temperatures (Tg) and adequate electrical properties. Unfortunately, however, the poor fracture toughness of epoxy resins oftentimes limits the usefulness thereof.
The impact strength as well as most other physical properties of crosslinked epoxy resins is controlled by the chemical structure and ratio of the epoxy resin and hardener, by any added fillers, and by the curing conditions used. Unfortunately, crosslinked, glassy epoxy resins with relatively high Tg(>100° C.) are brittle. The poor impact strength of high glass transition epoxy resins limits the usage of epoxies as structural materials and in composites.
Indeed, current commercially available underfill epoxy adhesives are excessively brittle and tend to fail prematurely in such applications as chip scale packaging (CSP) and related applications as a result of poor fracture toughness. Conventional toughening agents (e.g. carboxyl terminated butadiene, i.e., CTBN) are frequently unsuitable as additives in these adhesives because they adversely affect the capillary flow properties of the uncured adhesive.
Polysulfide-based toughening agents are known. For instance, polysulfide-based toughening agents of the general formulae:
where R is an alkyl ether (such as —(CH2)2—O—CH2—O—(CH2)2—) and a+b+c=n (where n is 6) and having about 2 mole percent branching, are known as THIOKOL LP-3, available commercially from Rohm and Haas Company, Philadelphia, Pa. LP-3 is reported to have a molecular weight of about 1,000.
In addition to the above-described commercially available materials, Akcros Chemicals (Manchester, Great Britain) also makes polysulfide-based toughening agents, which are available commercially under the tradename THIOPLAST, including products referred to as G1 (n is 19–21, 1.8–2 percent thiol content, and a 3,300–3,700 molecular weight), G4 (n is less than 7, less than 5.9 percent thiol content, and less than 1,100 molecular weight), G12 (n is 23–26, 1.5–1.7 percent thiol content, and a 3,900–4,400 molecular weight), G21 (n is 12–15, 2.5–3.1 percent thiol content, and a 2,100–2,600 molecular weight), G22 (n is 14–18, 2.1–2.7 percent thiol content, and a 2,400–3,100 molecular weight), G112 (n is 23–25, 1.5–1.7 percent thiol content, and a 3,900–4,300 molecular weight), and G131 (n is 30–38, 1.5–1.7 percent thiol content, and a 5,000–6,500 molecular weight). The THIOPLAST polysulfide-based toughening agents are reported to be prepared from the polycondensation of bis-(2-chloro-ethyl) formal with alkali polysulfide.
However, previously described polysulfide-based toughening agents are not useful in applications where good capillary flow is required, e.g., in underfill adhesive applications. Accordingly, there is a need for toughening agents that are effective for improving the toughness of adhesive formulations, especially in formulations requiring good capillary flow properties.